Surface I-A molecules from reticulum cell sarcomas (RCS) of SJL mice induce a strong proliferative response and lymphokine production in syngeneic Ly1+ 2- T cells but not in T cells from F1 hybrid mice of SJL with strains expressing I-E. Thymus and lymph node cells from bone marrow chimeras of nonresponder F1 into SJL respond as well as do SJL T cells and, conversely, SJL to F1 chimera cells fail to respond. It is not clear whether this is due to negative or positive influences on T-cell responses in the host thymus. However, it has been established that RCS cells do not express alien I-E-alpha chains, and monoclonal antibody to I-A completely blocks T cell stimulation by RCS. Two-dimensional gel electrophoresis of labeled surface I-A show acidic A-alpha chains or RCS cells absent from normal SJL spleen cells but at least partially present on LPS-induced B-cell blasts. The molecular heterogeneity with respect to charge but not size is removed by treatment with neuraminidase, whereas tunicamycin-treated RCS and SJL spleen cell I-A molecules appear identical. Neuraminidase treatment enhances, while tunicamycin treatment abolishes, the ability of RCS to stimulate syngeneic T cells. The results suggest an important role of glycosylated I-A molecules in the syngeneic T-cell stimulation. (LB)